We performed a comparative study between xenodiagnosis and serological tests for Chagas'disease. 150 Patients from several endemic areas were studied. Pour of them appeared to have a peculiar status with positive xenodiagnosis and negative serology carried out with four classical techniques (Immunofluorescence test, ELISA: Enzyme Linked Immunosorbent Assay, Complement fixation test and Immunoelectrophoresis). One serum out of the four patients presenting humoral depression showed a high quantity of circulating antigen proved by Immunoelectrophoresis. The Authors suggest the use of one serological test for detecting circulating antigens of Trypanosoma cruzi in addition to the classical serology. It would allow the diagnosis of Chagas' disease in patients with low production of specific antibodies.